In the distribution type fuel injection pump, the internal pressure (residual pressure) of an injection pipe from the delivery valve to the fuel injection nozzle is arranged to be kept at a constant pressure by the delivery valve. This is because of the fact that in case the pressure in the injection pipe is instable, irregular injection occurs, and if the residual pressure changes for each rotation, problems such as occurrence of lack of continuity of the injection characteristics occur.
A higher internal pressure (residual pressure) in the injection pipe results in a higher maximum internal pressure in the pipe and produces the merit of eliminating a phenomenon that the opening of the valve of the injection nozzle in a low speed zone (idling time) is at last effected by a secondary wave, or tertiary wave, not a primary wave, but inversely, there is a demerit that secondary injection occurs easily.
However, at present the residual pressure meeting both requirements is set by the delivery valve, and the problems of generation of secondary injection and instability of injection in the low speed zone have not been solved.